Download Beta-delayed two-neutron emission

-delayed two-neutron emission
Abstract
Very little is known on nucleon-nucleon correlations in the atomic
nucleus. One way to study these correlations is search for correlated
emission of two nucleons. The best probes from a theoretical point of
view is most likely the emission of two neutrons, as the neutrons are
not perturbed by the Coulomb barrier.
Keywords
Beta-delayed two-neutron emission, exotic decay modes, nuclear
pairing, single-particle levels
Physics case
When moving further and further away from the valley of stability, the Q values
for  decay increase more and more. Close to the drip line, -delayed particle
emission is observed. Even further away, -delayed two-nucleon emission can
be observed and studied. -delayed two-proton emission has been observed
experimentally for 9 different nuclei, but only the decay of 31Ar has been
studied to some extent. On the neutron-rich side, b-delayed two-neutron
emission has been evidenced for 7 nuclei with branching ratios ranging from
1% to 10%. However, for none of these nuclei, correlations between the two
neutrons have been searched for.
These correlations may yield valuable information about the pairing of
nucleons inside the atomic nucleus, which is not accessible otherwise. In
particular, two-neutron emission has a decisive advantage over two-proton
emission, which is that the two neutrons are not disturbed by the Coulomb
barrier and a possible correlation should be observable outside the nucleus.
Beyond their interest for correlation studies, the decay characteristics of these
nuclei are also of interest for the modeling of the astrophysical rapid-neutron
caption process.
Observables
To identify new two-neutron emitters and to study their decays, these
isotopes have to be implanted into a catcher, which is surrounded by a highefficiency, high-granularity neutron detection system. The observables to be
measured are the half-life of the nucleus, the energy of the neutrons and in
particular the angle between the two neutrons. In addition, to complete the 
decay scheme,  radiation should also be observed.
Proposed experiment
The known 2n emitters are 11Li, 17B, 17C, 30,31Na, 32,33Na, but many
others are expected close to the neutron drip line. These isotopes can be
produced e.g. by fragmentation reactions or by deep inelastic reactions. After
implantation in the center of the detection setup, the neutrons will be detected
and their energy and angular correlation will be determined.
Requirements
Beam properties
High-intensity neutron-rich stable or radioactive beams at about 100-150
MeV/nucleon for fragmentation or deep inelastic reactions
Detection
To produce the above mentioned isotopes by projectile fragmentation, a
high-resolution, high-acceptance fragment separator is needed to separate the
exotic species from the bulk part of less exotic nuclei. At the end of this
separator, a high-efficiency, high-granularity neutron setup is needed to give
access to the individual energies of the two neutrons and their emission angle.
Theoretical support
The interpretation of the experimental data requires sophisticated
theoretical models which describe coherently the nuclear structure part
and the nuclear dynamics of the emission process. Most of the models
available today treat only one of the two parts reasonably well. These
models have to be refined and new concepts like time-dependent
approaches have to be implemented. In particular, the experimental
observables to study e.g. the pairing force are not very well established.